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Chapter 18 | Electric Charge and Electric Field
61. Integrated Concepts
Figure 18.57 shows an electron passing between two charged metal plates that create an 100 N/C vertical electric field perpendicular to the electron's original horizontal velocity. (These can be used to change the electron's direction, such as in an oscilloscope.) The initial speed of the electron is
, and the horizontal distance it travels in the
uniform field is 4.00 cm. (a) What is its vertical deflection? (b) What is the vertical component of its final velocity? (c) At what angle does it exit? Neglect any edge effects.
63. Integrated Concepts
(a) In Figure 18.59, four equal charges lie on the corners
of a square. A fifth charge is on a mass directly above
the center of the square, at a height equal to the length of one side of the square. Determine the magnitude of in
terms of , , and , if the Coulomb force is to equal the weight of . (b) Is this equilibrium stable or unstable? Discuss.
Figure 18.59 Four equal charges on the corners of a horizontal square support the weight of a fifth charge located directly above the center of the square.
64. Unreasonable Results
(a) Calculate the electric field strength near a 10.0 cm diameter conducting sphere that has 1.00 C of excess charge on it. (b) What is unreasonable about this result? (c) Which assumptions are responsible?
65. Unreasonable Results
(a) Two 0.500 g raindrops in a thunderhead are 1.00 cm apart when they each acquire 1.00 mC charges. Find their acceleration. (b) What is unreasonable about this result? (c) Which premise or assumption is responsible?
66. Unreasonable Results
A wrecking yard inventor wants to pick up cars by charging a 0.400 m diameter ball and inducing an equal and opposite charge on the car. If a car has a 1000 kg mass and the ball is to be able to lift it from a distance of 1.00 m: (a) What minimum charge must be used? (b) What is the electric field near the surface of the ball? (c) Why are these results unreasonable? (d) Which premise or assumption is responsible?
Figure 18.57
62. Integrated Concepts
The classic Millikan oil drop experiment was the first to obtain an accurate measurement of the charge on an electron. In it, oil drops were suspended against the gravitational force by a vertical electric field. (See Figure 18.58.) Given the oil drop to
be in radius and have a density of :
(a) Find the weight of the drop. (b) If the drop has a single excess electron, find the electric field strength needed to balance its weight.
Figure 18.58 In the Millikan oil drop experiment, small drops can be suspended in an electric field by the force exerted on a single excess electron. Classically, this experiment was used to determine the electron charge by measuring the electric field and mass of the drop.
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